The present invention relates to a method for calibrating radiometric hygrometers or moisture measuring instruments. The moisture content in loosely piled and heaped material such as coke is presently measured with instruments based on nuclear physics and phenomena. For example, moisture can be determined by using the moderation of fast neutron by hydrogen nuclei (see, for example, Stahl und Eisen, Vol. 82, (1962), pages 1017 to 26). Such a moisture detecting instrument includes a source of neutron radiation such as 241-americium-beryllium and a scintillometer is placed in the immediate vicinity of the source and detector. Source and detector are constructed as probe inside of a protective pipe which, in turn, is placed in the storage facility (container, bin, silo etc.) for the loose moist material. Particularly, the detector is connected to an amplifier and a recorder by means of a special cable.
Upon calibrating this instrument, one usually proceeds as follows. Generally, the instrument is calibrated on basis of otherwise, e.g. empirically available date on the moisture content of the material, gained, for example, by gravimetrically testing such material. Calibrating includes zero offset and gain for the amplification of the detector pulses. The adjustment of zero level and offset includes the instrument parameters as well as the hydrogen content of the material itself, other than in water. Also, the so-called null effect of the probe is induced here.
The instrument characteristics which represent the mathematical relation between instrument incidation and reading on one hand, and the actual moisture content, is determined by determining the moisture content independently, e.g. by gravimetrically analyzing samples. These gravimetric data are then associated with radiometric readings supposedly involving material with the same moisture content as the samples.
The samples are obtained as follows. The container for the loose material is filled so that upon reaching a volume later to be monitored by the calibrated instrument (measuring volume), samples of the material can actually be taken, e.g. through branching off some of the flow and in the order of several pounds material. As the container is filled up to the measuring volume, a reading is taken from the instrument, while the samples are subjected to direct moisture content analysis. Since the mathematical relation between actual moisture content and reading is not umambiguous, many readings, i.e. several hundred readings, spread over several months have to be taken and to be subjected to a regression analysis. These prior art methods and calibration is described by J. Schwarzlose in "Isotope Praxis", 3rd year, 1967, issue 9, the article being entitled "Feuchtemessung an Stuckkoks mit Neutronen" (moisture measurement for lump coke with neutrons) and by J. Luckers, C.N.R.M., No. 11. June 1967 "Continuous Measurement of the Moisture Content of Coke by Means of a Neutron Probe".
In view of the large scattering of the measured values, even a regression analysis will yield only rather vague information of the instrument characteristics. The reason for the ambiguity and scattering of readings lies usually in the method of taking samples. The moisture content of the material is not uniform and upon taking random samples one may have missed that portion which contributes most to the inhomogeneity in the moisture content, but which may significantly influence the radiometric reading. In other words, the samples are not really representative of the moisture content of the measuring volume or of the bin content as a whole. Also of disadvantage is that the measuring points crowd around a medium reading value and are not distributed over the entire measuring range, even though the actual average moisture content in the bin varies. The reason for scattering of the measuring points is also treated in the said two references.
The resulting instrument characteristics was found to be rather unreliable; still, it is then transformed into a calibration curve or calibrated instrument characteristics by means of generally known mathematical and measuring technique methods. This curve is then supposed to represent a definite and exact relation between radiometric moisture detection and the true moisture values, and is transposed upon the test and measuring instrument.
In order to supervise the instrument as to long term stability (drift), one will occasionally run the instrument in a simulated moisture environment (moisture phantom). In order to have a reference available, this simulated environment is established prior to installation of the instrument, i.e., during calibration. One will use here a material such as paraffin wax, a boron paraffin mixture or the like, and the chemical properties thereof will lead to a reading which is separately recorded as a calibration reading to be used as a reference later. The simulated environment is re-established (or maintained) and is used whenever long term drift is to be detected; the reading then obtained is compared with the earlier reference.
This method will yield only one indication as to long term drift of the characteristics as a whole and manifests itself as a parallel shift. Any rotation of the curve will not be detected. One would need here several, at least three such long term reference and calibration points, i.e. one would need correspondingly different simulations. A single phantom condition can yield only one reference point, and cannot be used as standard for calibrating the entire range of the instrument (or of several instruments operating under similar conditions). It was found also that the phantom is actually quite unreliable because that single reference point has about the same degree of uncertainty as the calibration curve itself.
The known methods for calibrating a moisture detecting instrument are, therefor, as follows:
1. The entire procedure is quite time consuming;
2. The procedure must be repeated for each instrument;
3. Sufficiently exact and reliable checks after repairs are lacking;
4. Reproducing results are imperfect, because;
5. the readings scatter, not because of the instrument itself, but because the calibrating samples are not representative for the measuring volume as a whole;
6. The mathematical transformation becomes uncertain when the calibration measurements deviate too much from the final calibration curve;
7. Human errors in taking samples and introduction of subjective errors, particularly when calibration extends over long periods of time.